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The Anatomy of Botany: Woods Go Their Own Ways

April 18, 1998|From Associated Press

All trees belong in one of two broad divisions: hardwoods or softwoods.

Yet when it comes to characteristics, distinctions between the two tend to blur. Certain softwoods, such as southern yellow pine, are harder than some hardwoods, such as basswood. Because hardness and strength go hand-in-hand, the strongest woods are also the most difficult to work and require the sharpest tools. Such dense woods are also less forgiving if the joinery is carelessly done.

All woods have grain, a term that describes the direction of longitudinal cells in a board. Relative cell size, which determines whether a wood needs a filler before finishing, is called texture. The attractive patterns of various boards, called figure, are caused by a deviation from the tree's normal growth.

Freshly sawn wood has a high moisture content and should be seasoned, or dried, before working. Lumberyards usually sell kiln-dried wood with a moisture content between 6% and 18%. Wood that has less than 10% moisture content is recommended for furniture-making. The range above 10% is suitable for structures.

Even after seasoning, wood continues to shrink during dry spells and swell with humidity. The tendency is critical to the woodworker because it can cause warped boards, loose joints or swollen-shut drawers. To combat this movement, you can choose a wood with strong stability. Ash, basswood, incense cedar, mahogany, white pine, redwood, rosewood, teak and walnut are very stable woods.

Woods with good stability include beech, butternut, black cherry, hickory, red oak and poplar. Yellow birch and sugar maple are considered fairly stable.

The easiest woods to work are basswood, butternut, black cherry, mahogany, red oak, white pine, poplar, redwood, teak and walnut. However, red oak can splinter, redwood has a tendency to split and crack when nailed, and teak is hard to glue.

Wood dries more quickly along the grain than across it. The resulting uneven moisture loss causes cracks, called checks, on board ends. Wood also distorts when it shrinks or swells unevenly. The stress produced in the fibers causes deviations--such as cupping, bowing or twisting--from the board's flatness.

Buy kiln-dried boards in advance and store them indoors for about a month to acclimate them to the moisture content of your home. To ensure good air circulation and to minimize warp, separate layers of boards with 1-by-1 strips of wood. Place the strips at each end of the stack and about every 16 inches along the lengths of the boards. After working the wood, apply a sealing finish on all surfaces to retard moisture exchange.

Wood comes in various grades. In hardwood, "first and second" (FAS) boards are about 85% defect-free. For softwood projects, choose grades A and B for fine finishing and C and D for painting.

Most hardwood is sold by the board foot, a unit 1 foot long, 1 foot wide and 1 inch thick (144 cubic inches). Softwoods are often sold by the running or linear foot, which refers to the board's length.

Dimensional lumber (2-by-4s, 2-by-6s) is sold in its nominal size, the dimension before sawing or planing takes place, or in its smaller actual size, the size after surfacing. Softwood sold in dimensional sizes loses one-half-inch in each dimension. Hardwood loses about one-quarter-inch in thickness.

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